There are at least three bands/performers who go by the name Callaghan.

1. Callaghan http://www.callaghansongs.com is a British acoustic/pop singer, originally from Lincolnshire and now performing in London and Nashville.
Born in Boston, Lincolnshire to a musical family, Callaghan was classically trained on flute from age 6, then gravitated toward singing and playing piano. By 14 she was writing songs, and started trekking to London to record with various producers. At 18 Callaghan relocated to London permanently to live with her sister (whose flat was so tiny, the singer slept under the kitchen table), while recording a number of independent EPs and playing all over the city, at spots like the Bedford, Regal Room, Royal National Theatre, and 100 Club. A long-time Mullins fan, Callaghan contacted him in 2009 through MySpace while seeking a producer, and he agreed to a rare collaboration. An initial recording trip to Atlanta yielded three songs, prompting her life-changing relocation there in 2010 to finish the full-length.

2. Callaghan http://www.myspace.com/callaghanrock is a Spanish Rock group based in Madrid.

3. Callaghan is a punk band from Chicago, IL. Their first (self-titled) album was released on (now-defunct) Seven Dead Arson. There is an EP, Fired a Blank, available from Red Line distribution.
http://www.redlinedistribution.com/interviews/Callaghan.shtml